Capacitor charging apparatus

ABSTRACT

A capacitor charging apparatus includes a transformer and an output capacitor charged with current flowing through a secondary coil of the transformer, and charges the output capacitor by performing a switching control of a switching transistor provided on a path leading to a primary coil of the transformer. A switching control unit controls on and off of the switching transistor. A voltage detector monitors a voltage at a tap provided in the secondary coil of the transformer. The switching control unit regards the voltage detected by the voltage detector as an output voltage of the capacitor charging apparatus, and controls the on and off of the switching transistor.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. patentapplication Ser. No. 11/714,429, filed on Mar. 6, 2007, the entirecontents of which are incorporated herein by reference. The Ser. No.11/714,429 application claimed the benefit of the dates of Mar. 7, 2006to Japanese Application Nos. JP2006-061686; JP2006-061689; andJP2006-061692; and of Jul. 5, 2006 to Japanese Application No.JP2006-186099, the entire contents of which are incorporated herein byreference, and priority which is hereby claimed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a switching power supply and, moreparticularly, to a capacitor charging apparatus for generating highvoltage by charging a capacitor.

2. Description of the Related Art

In a variety of electronic equipment, a step-up switching power supplyis used to supply a voltage higher than the input voltage to a load.Such a step-up switching power supply, which has a switching element anda transformer, outputs a boosted input voltage by producing a backelectromotive force in the transformer by turning the switching elementon and off in a time-division manner and charging an output capacitorwith the current flowing through the secondary coil of the transformer.

An input voltage is applied to one end of the primary coil of thetransformer, and the switching element is coupled to the other endthereof. The voltage at one end of the secondary coil of the transformeris fixed, and an output capacitor is coupled to the other end thereofvia a rectifier diode.

With such a switching power supply, when a switching transistor,provided as the switching element, turns on, a current flows through theprimary winding of the transformer, and energy is stored in thetransformer. Then, as the switching transistor turns off, the energystored in the transformer is supplied from the secondary side thereofand transferred as a charging current to the output capacitor via therectifier diode. The output capacitor is thus charged by the repetitionof on and off of the switching transistor, and the output voltage rises.

References (1) to (3) listed in the following Related Art List, forexample, disclose control circuits for their respective self-excitedcapacitor charging devices that control the on and off of the switchingtransistor according to the monitored state of the primary and/or thesecondary side of the transformer.

RELATED ART LIST

-   (1) Japanese Patent Application Laid-Open No. 2003-79147.-   (2) U.S. Pat. No. 6,518,733.-   (3) U.S. Pat. No. 6,636,021.

Problem 1: A capacitor charging apparatus sometimes controls the circuitoperation according to an output voltage appearing at the outputcapacitor. For example, the detection of the completion of charging,that is, whether an output voltage sufficient to drive a load has beengenerated or not, is carried out by monitoring the output voltage.According to Reference (1) above, for instance, the completion ofcharging is detected by monitoring the voltage across the primary coilof a transformer as a way of monitoring the output voltage indirectly.

However, since the correlation between the voltage across the primarycoil of a transformer and the output voltage can vary with the turnsratio of the transformer or other conditions, such an arrangementpresents a problem of inability to detect the output voltage accurately.As a result, there may be cases where a drive voltage sufficient todrive the load is not obtained or an overcharging beyond the necessaryvoltage occurs to consume electric power wastefully.

Also, where the output voltage appearing at the output capacitor ismonitored directly by dividing it with resistors, it is necessary to useresistive elements that is capable of withstanding high voltage. Andsuch a resistive elements capable of withstanding high voltage must beinstalled as a chip because it is difficult to incorporate it within anLSI. This results in a larger number of circuit components and thus alarger packaging area. Moreover, there may be cases where a diode mustbe installed backward to prevent the charge stored in the outputcapacitor from discharging to the ground via the resistive element.

Problem 2: The applicant of the present invention has come to realizethe following problems in an investigation on a capacitor chargingapparatus so configured that it can adjust the amount of chargingcurrent for the output capacitor.

If the charging current is varied at the capacitor charging apparatus,the current flowing through the switching element, which is provided toturn on and off the primary coil of the transformer and the primarycurrent of the transformer, will also vary.

For example, when a bipolar transistor is used as the switching element,a base current must be intermittently supplied as a switching signal tothe base thereof to turn it on and off. In this case, if the amplitudeof the base current is fixed independent of the charging current (i.e.,the collector current of the bipolar transistor), then there may becases where current is spent wastefully or sufficient charging currentcannot be obtained. The collector current of a bipolar transistor issubject to the influence of base current, and if more than necessaryamount of base current is supplied for the required charging current(collector current), the excess base current will result in a wastefulconsumption of electric power. Conversely, if less than necessary amountof base current is supplied for the required charging current, thecollector current generated will be less than adequate, thus hinderingthe charging operation.

A similar problem may also arise when a MOSFET (Metal OxideSemiconductor Field Effect Transistor) is used as the switching element.That is, the drain current of a MOSFET, which corresponds to thecharging current, is subject to the influence of the gate voltagethereof, and if the amplitude of the gate voltage or the drive currentrelative to the gate capacitance is fixed independent of the chargingcurrent (drain current), then there may be cases where current is spentwastefully or sufficient charging current cannot be obtained.

Problem 3: The timing at which the switching transistor turns on and offin such a capacitor charging apparatus is a very important technicalissue that influences the efficiency and the charging rate, which aregenerally in a trade-off relation with each other. For instance, theabove-mentioned References (1) to (3) disclose control methods in whichthe on-timing of the switching transistor is determined according to thecurrent flowing through the primary coil of a transformer and theoff-timing of the switching transistor is determined according to thecurrent flowing through the secondary coil thereof.

According to the method described in the above References, the switchingtransistor turns on at the timing when the current flowing through thesecondary coil becomes sufficiently small, namely, at the timing whenthe energy stored in the transformer is discharged to an outputcapacitor. This method can enhance the utilization efficiency of energy.However, since in this method an off-time during which the switchingtransistor is turned off is determined by the energy stored in thetransformer, there is a problem that the output capacitor cannot becharged rapidly.

SUMMARY OF THE INVENTION

1. An advantage of one embodiment of the present invention is to providea capacitor charging apparatus capable of accurately detecting outputvoltage by use of a simple circuit structure.

One embodiment of the present invention relates to a control circuit ofa capacitor charging apparatus, the capacitor charging apparatusincluding a transformer and an output capacitor charged with currentflowing through a secondary coil of the transformer, which charges theoutput capacitor by performing a switching control of a switchingtransistor provided on a path leading to a primary coil of thetransformer. This control circuit comprises: a switching control unitwhich controls on and off of the switching transistor; and a voltagedetector which monitors a voltage at a tap provided on the secondarycoil of the transformer. The switching control unit controls the on andoff of the switching transistor by taking the voltage detected by thevoltage detector as an output voltage of the capacitor chargingapparatus.

A tap (F winding) is provided on the secondary of the transformer andthe potential thereof is monitored, so that it is possible to detectwith accuracy the voltage in response to the output voltage appearing atthe capacitor. Also, the location of the tap is on the grounding side,namely the lower-voltage side, of the secondary coil. This arrangementmakes it unnecessary to use resistance voltage division, thus avoidingany increase in the number of circuit components used. That is, thecontrol circuit according to this embodiment can optimally performswitching control by monitoring the output voltage with accuracy.

The switching control unit may include a charging completion detectorwhich detects completion of charging by comparing the voltage detectedby the voltage detector with a predetermined threshold voltage. And whena completion of charging is detected, the switching control unit maystop the on and off of the switching transistor.

As described above, the voltage of the tap detected by the voltagedetector is equal to a voltage corresponding to the output voltage.Hence, compared with the direct monitoring of the output voltageappearing at the output capacitor, this arrangement makes it possible tostop the switching by detecting a full charging state without any lossin accuracy.

The switching control unit may repeat an operation of turning on theswitching transistor until current flowing through the primary coil ofthe transformer has reached a predetermined peak current and of thenturning off the switching transistor during an off-time. In so doing,the off-time may be varied in response to the voltage detected by thevoltage detector. The switching control unit may set the off-time insuch a manner that the higher the voltage detected by the voltagedetector is, the shorter the off-time becomes.

In such a case, the output voltage can be monitored with accuracy. Thus,the off-time of the switching transistor, namely the time during whichthe charging current is supplied to the output capacitor can beoptimally controlled.

The switching control unit may include: a primary-current detectioncircuit which detects a primary current flowing through the primary coilof the transformer; a comparator which compares the primary currentdetected by the primary-current detection circuit with a predeterminedpeak current value and outputs a comparison signal of a predeterminedlevel when the primary current exceeds the predetermined peak currentvalue; an off-time setting circuit which begins to count an off-timewhen the comparison signal outputted from the comparator becomes thepredetermined level; and a driver circuit which repeats an operation ofturning off the switching transistor during a period of time in whichthe off-time is counted and turning on the switching transistor afterthe off-time has elapsed. The off-time setting circuit may vary theoff-time in accordance with the voltage detected by the voltagedetector. The off-time setting circuit may set the off-time in such amanner that the higher the voltage detected by the voltage detector is,the shorter the off-time is.

A control circuit according to an embodiment may be integrated on asingle semiconductor substrate. “Being integrated” includes a case whereall of circuit components are formed on a semiconductor substrate and acase where the main components of a circuit are integrated thereon. Notethat part of resistors or capacitors used to adjust circuit constantsmay be provided outside the semiconductor substrate. Integrating thecontrol circuit into a single LSI can reduce the circuit area.

Another embodiment of the present invention relates to a capacitorcharging apparatus. This apparatus comprises: a transformer, including aprimary coil and a secondary coil, wherein an input voltage is appliedto one end of the primary coil and a switching transistor is connectedto other end thereof; an output capacitor one end of which is grounded;a diode having an anode thereof connected to a second coil side of thetransformer and a cathode thereof connected to the other-end side of theoutput capacitor; and a control circuit, according to any one of theabove-described embodiments, which controls on and off of the switchingtransistor.

According to this embodiment, the switching control can be performed bydetecting with accuracy the output voltage appearing at the outputcapacitor. Also, the packaging area can be reduced.

Still another embodiment of the present invention relates to a lightemitting apparatus. This apparatus comprises: the above-describedcapacitor charging apparatus; and a light-emitting element driven by anoutput voltage appearing at the output capacitor in the capacitorcharging apparatus. According to this embodiment, the packaging area andthe number of circuit elements used for the capacitor charging apparatusare reduced, so that the apparatus can be easily mounted on a small-sizeset.

Still another embodiment of the present invention relates to anelectronic apparatus. This electronic apparatus comprises: theabove-described light emitting apparatus; and a control unit whichcontrols an emitting state of the light emitting apparatus. According tothis embodiment, the packaging area and the number of circuit elementsused for the capacitor charging apparatus are reduced, so that the sizeof a casing for a set can be made smaller.

Still another embodiment of the present invention relates to a methodfor controlling a capacitor charging apparatus, including a transformerand an output capacitor charged with current flowing through a secondarycoil of the transformer, which charges the output capacitor byperforming a switching control of a switching transistor provided on apath leading to a primary coil of the transformer. This control methodcomprises: repeating: a first step of detecting a primary currentflowing through the primary coil of the transformer; a second step ofturning on the switching transistor until the detected primary currentreaches a predetermined peak current; and a third step of turning offthe switching transistor during an off-time wherein the third step comesafter the second step; and a fourth step of detecting that a voltage ata tap provided on the secondary coil of the transformer has reached apredetermined threshold voltage. When the voltage at the tap hasexceeded the predetermined threshold voltage, the repeating the first tothe third step is stopped. In the third step the off-time may be set inaccordance with the voltage at the tap provided on the secondary coil ofthe transformer.

2. Another advantage of embodiments of the present invention describedhereinafter is to provide a control circuit capable of optimally drivinga switching transistor in a capacitor charging apparatus where chargingcurrent is adjustable.

One embodiment of the present invention relates to a control circuit ofa capacitor charging apparatus, the capacitor charging apparatusincluding a transformer and an output capacitor charged with currentflowing through a secondary coil of the transformer, which charges theoutput capacitor by performing a switching control of a switchingtransistor provided on a path leading to a primary coil of thetransformer. This control circuit comprises: a primary-current detectioncircuit which detects a primary current flowing through the primary coilof the transformer; and a switching control unit which monitors at leastthe primary current detected by the primary-current detection circuitand outputs to a control terminal of the switching transistor aswitching signal for specifying ON to the switching transistor until theprimary current reaches a predetermined peak current value and thenspecifying OFF to the switching transistor during an off-time. Theswitching control unit adjusts the switching signal outputted to thecontrol terminal of the switching transistor, according to thepredetermined peak current value.

According to this embodiment, the charging current is defined by apredetermined peak current value. With the structure in this embodiment,the switching signal most suitable for the on and off of the switchingtransistor can be supplied by adjusting the switching signal supplied toa control terminal of the switching transistor according to thepredetermined peak current value.

The switching transistor may be a bipolar transistor, and the switchingcontrol unit may adjust a current value of a base current, which issupplied to a base of the bipolar transistor as the switching signal, inaccordance with the predetermined peak current value. The switchingcontrol unit may set the current value of a base current in such amanner that the larger the predetermined peak current value is, thelarger the current value of a base current becomes. Furthermore, theswitching control unit may set the current value of a base current insuch a manner the current value of a base current is proportional to thepredetermined peak current value.

In such a case, the base current of the switching transistor is definedaccording to the charging current defined by the predetermined peakcurrent value. As a result, the base current most suitable according toa collector current of the switching transistor can be supplied andtherefore the efficiency of circuitry can be enhanced.

The switching control unit may include a current generator whichreceives a current adjustment signal specifying the predetermined peakcurrent value and generates a current in accordance with the currentadjustment signal, and the current generated by the current generatormay be supplied to a base of the switching transistor as a switchingsignal in a period during which the switching transistor is to turn on.

The current adjustment signal may be inputted to a charge currentcontrol terminal provided in the control circuit, from outside thecontrol circuit. With the structure in this embodiment, the chargingcurrent can be controlled from the outside and at the same time thecontrol signal of the switching transistor can be adjusted according toa charging current set from the outside.

The switching control unit may monitor an output voltage appearing atthe output capacitor and then adjust the off-time according to thisoutput voltage. The output voltage may be monitored directly orindirectly. In such a case, when the output voltage immediately after astart of charging is low, the energy stored in the transformer can beused efficiently; and as the output voltage gets higher, the chargingrate can be raised, so that the efficiency and the charging rate can bebalanced.

Another embodiment of the present invention relates also to a controlcircuit of a capacitor charging apparatus, the capacitor chargingapparatus including a transformer and an output capacitor charged withcurrent flowing through a secondary coil of the transformer, whichcharges the output capacitor by performing a switching control of aswitching transistor provided on a path leading to a primary coil of thetransformer. This control circuit comprises: a current generator whichgenerates a current according to the amount of a charging current forthe output capacitor; and a driver circuit which supplies the currentgenerated by the current generator as a base current of a bipolartransistor serving as the switching transistor, in a period during whichthe switching transistor is to turn on. The current generator may setthe current value of a base current in accordance with a peak value ofthe charging current.

The control circuit in the capacitor charging apparatus may beintegrated onto a single semiconductor substrate. “Being integrated”includes a case where all of circuit components are formed on asemiconductor substrate and a case where the main components of acircuit are integrated thereon. Note that part of resistors orcapacitors used to adjust circuit constants may be provided outside thesemiconductor substrate. Integrating the control circuit into a singleLSI can reduce the circuit area.

Still another embodiment of the present invention relates to a capacitorcharging apparatus. This apparatus comprises: a transformer, including aprimary coil and a secondary coil, wherein an input voltage is appliedto one end of the primary coil and a switching transistor is connectedto other end thereof; an output capacitor one end of which is grounded;a diode having an anode thereof connected to a secondary coil side ofthe transformer and a cathode thereof connected to the other-end side ofthe output capacitor; and the above-described control circuit whichcontrols on and off of the switching transistor.

According to this embodiment, the charging current for the capacitor canbe optimally controlled and the driving state of the switchingtransistor can be optimized according to the charging current. Thus thecurrent consumption can be reduced.

Still another embodiment of the present invention relates to a lightemitting apparatus. This apparatus comprises: the above-describedcapacitor charging apparatus; and a light-emitting element driven by anoutput voltage appearing at the output capacitor in the capacitorcharging apparatus. According to this embodiment, high voltage requiredfor driving the light-emitting element can be generated mostefficiently.

Still another embodiment of the present invention relates to anelectronic apparatus. This electronic apparatus comprises: theabove-described light emitting apparatus; and a control unit whichcontrols an emitting state of the light emitting apparatus.

3. Still another advantage of embodiments of the present inventiondescribed hereinafter is to provide a capacitor charging apparatus wherethe charging rate and the efficiency are balanced.

One embodiment of the present invention relates to a control circuit ofa capacitor charging apparatus, the capacitor charging apparatusincluding a transformer and an output capacitor charged with currentflowing through a secondary coil of the transformer, which charges theoutput capacitor by performing a switching control of a switchingtransistor provided on a path leading to a primary coil of thetransformer. The control circuit comprises: an off-signal generatorwhich monitors current flowing through the primary coil of thetransformer and which outputs an off signal of a predetermined levelwhen the current rises up to a predetermined peak current; a firston-signal generator which monitors voltage across the primary coil ofthe transformer and which outputs a first on signal of a predeterminedlevel when the voltage across the primary coil thereof drops down to afirst predetermined threshold voltage; a second on-signal generatorwhich monitors a monitoring voltage corresponding to an output voltageappearing at the output capacitor and sets an off-time based on themonitoring voltage and which outputs a second on signal of apredetermined level after the off signal of the predetermined level hasbeen outputted from said off-signal generator and then the off-time haselapsed; and a switching control unit which receives the off signal fromthe off-signal generator and receives the first and the second on signalfrom the first and the second on-signal generator and which turns offthe switching transistor according to the off signal and turns on theswitching transistor according to the first and the second on signal.

In this embodiment, the energy is stored in the transformer with theswitching transistor being on. The off-signal generator determines theenergy stored in the transformer, by setting a peak value of the currentflowing through the primary coil of the transformer. With the switchingtransistor being off, the charging current flows from the secondary coilof the transformer toward the output capacitor. The energy stored in thetransformer is transferred with time. When the energy has beentransferred, the voltage across the primary coil attenuates withdamping. When the voltage across the primary coil drops down to thefirst predetermined threshold voltage, the first on-signal generatordetects that the energy has been completely expended, and determines thenext on-timing of the switching transistor. The second on-signalgenerator sets a period, during which the switching transistor is toturn off, and also sets the next on-timing of the switching transistoraccording to the output voltage irrespective of the energy stored in thetransformer.

According to this embodiment, the energy to be stored in thetransformer, namely the charging rate, can be set by the off-signalgenerator. At the same time, the on-timing of the switching transistoris set according to the on signal outputted from either one of the firston-signal generator and the second on-signal generator, so that theselection as to whether priority is given to the efficiency or chargingrate can be adjusted.

When the off signal of the predetermined level is outputted from theoff-signal generator, the switching control unit may turn off theswitching transistor; and the switching control unit may turn on theswitching transistor according to the on signal, whichever first goes tothe predetermined level, of either the first on signal outputted fromthe first on-signal generator or the second on signal outputted from thesecond on-signal generator.

The switching control unit may be configured in a manner that a mode ofoperation is selectable between a first mode that operates according tothe first on signal and the off signal and a second mode that operatesaccording to the second on signal and the second off signal. In such acase, the mode can be switched between a mode where the efficiency isgiven priority and a mode where the charging rate is given priority,according to application, a condition where the capacitor chargingapparatus is used, or the like.

When the monitoring voltage is less than or equal to a secondpredetermined threshold voltage, the switching control unit may turn onthe switching transistor according to the second on signal.

When the output voltage appearing at the output capacitor is low, thevoltage across the primary coil of the transformer gets smaller, so thatthere is a possibility that the first on signal be generated by mistakedue the effect of noise. In the light of this, the switching transistoris turned on according to the second on signal if the output voltage islow. Thereby, the switching transistor can remain off appropriately.

The second on-signal generator may set the off-time in a manner that avoltage corresponding to a voltage appearing at a tap provided in thesecondary coil of the transformer is taken as the monitoring voltage. Atthe tap provided in the secondary coil of the transformer, there appearsa voltage equal to the output voltage multiplied by a ratiocorresponding to the number of turns. Hence, the off-time can beappropriately set according to the output voltage.

The switching control unit may include a flip-flop which is set by theoff signal and reset by either the first on signal or the second onsignal, and the switching control unit may control on and off of theswitching transistor according to an output signal of the flip-flop.

The on-signal generator unit may set the off-time in such a manner thatthe larger the monitoring voltage is, the shorter the off-time becomes.In this case, when the output voltage immediately after a start ofcharging is low, the energy stored in the transformer can be usedefficiently. And as the output voltage gets higher, the charging ratecan be raised. Thus, the efficiency and the charging rate can bebalanced.

A control circuit according to an embodiment may be integrated on asingle semiconductor substrate. “Being integrated” includes a case whereall of circuit components are formed on a semiconductor substrate and acase where the main components of a circuit are integrated thereon. Notethat part of resistors or capacitors used to adjust circuit constantsmay be provided outside the semiconductor substrate. Integrating thecontrol circuit into a single LSI can reduce the circuit area.

Another embodiment of the present invention relates to a capacitorcharging apparatus. This capacitor charging apparatus comprises: atransformer, including a primary coil and a secondary coil, wherein aninput voltage is applied to one end of the primary coil and a switchingtransistor is connected to other end thereof; an output capacitor oneend of which is grounded; a diode having an anode thereof connected to asecondary coil side of the transformer and a cathode thereof connectedto the other-end side of the output capacitor; and a control circuit,according to any one of the above-described embodiments, which controlson and off of the switching transistor.

Still another embodiment of the present invention relates also to acapacitor charging apparatus. This capacitor charging apparatuscomprises: a transformer, including a primary coil and a secondary coil,wherein one end of the primary coil is connected in common with one endof the secondary coil and an input voltage is applied to a commonconnection point thereof; a switching transistor connected to theother-end side of the primary coil; a diode having an anode thereofconnected to other end of the secondary coil; an output capacitorprovided between a cathode of the diode and ground; and a controlcircuit which controls on and off of the switching transistor.

According to this capacitor charging apparatus, the transformer isconfigured to have three terminals, so that the circuit area can bereduced.

In an embodiment, a tap may be provided in the secondary coil of thetransformer. The control circuit may perform at least part of on-offcontrol of the switching transistor, based on a voltage corresponding toa voltage appearing at the tap.

At the tap provided in the secondary coil of the transformer, thereappears a voltage equal to the output voltage multiplied by a ratiocorresponding to the number of turns. Hence, the switching controlaccording to the output voltage can be realized.

The control circuit may detect completion of charging of the outputcapacitor by comparing the voltage corresponding to a voltage appearingat the tap with a predetermined threshold voltage. In so doing, thepredetermined threshold voltage may be varied according to the inputvoltage.

The voltage appearing at the tap varies according to the input voltage.According to this embodiment, such a variation can be corrected.

The control circuit may include: a difference voltage generation circuitwhich generates a voltage corresponding to a difference voltage betweenthe voltage appearing at the tap and a voltage obtained by multiplyingthe input voltage by a predetermined constant; and a comparator whichcompares an output voltage of the difference voltage generation circuitwith a predetermined threshold voltage. Completion of charging may bedetected by the control circuit when the output voltage of thedifference voltage generation circuit exceeds the predeterminedthreshold voltage.

According to this embodiment, even if the input voltage varies, theoutput voltage of the capacitor charging apparatus at the time ofcompletion of charging thereof can be kept at a constant value.

The control circuit may include: an off-signal generator which monitorscurrent flowing through the primary coil of the transformer and whichoutputs an off signal of a predetermined level when the current rises upto a predetermined peak current; a first on-signal generator whichmonitors voltage across the primary coil of the transformer and whichoutputs a first on signal of a predetermined level when the voltageacross the primary coil thereof drops down to a first predeterminedthreshold voltage; a second on-signal generator which monitors amonitoring voltage corresponding to an output voltage appearing at theoutput capacitor and sets an off-time based on the monitoring voltageand which outputs a second on signal of a predetermined level after theoff signal of the predetermined level has been outputted from theoff-signal generator and then the off-time has elapsed; and a switchingcontrol unit which receives the off signal from the off-signal generatorand receives the first and the second on signal from the first and thesecond on-signal generator and which turns off the switching transistoraccording to the off signal and turns on the switching transistoraccording to the first and the second on signal.

A tap may be provided on the secondary coil of the transformer, and thesecond on-signal generator may monitor a voltage corresponding to avoltage appearing at the tap, as the monitoring voltage.

Still another embodiment of the present invention relates to atransformer. This transformer includes a primary coil and a secondarycoil, wherein one end of the primary coil is connected in common withone end of the secondary coil and a terminal is provided for a commonconnection point thereof, and terminals are further provided for theprimary coil and the secondary coil, respectively. According to thisembodiment, the transformer can be configured by three terminals insteadof the conventional four terminals, thereby realizing a smallercircuitry.

Still another embodiment of the present invention relates to a lightemitting apparatus. This apparatus comprises: a capacitor chargingapparatus according to any one of the above-described embodiments; and alight-emitting element driven by an output voltage appearing at theoutput capacitor in the capacitor charging apparatus.

Still another embodiment according to the present invention relates toan electronic apparatus. This electronic apparatus comprises: theabove-described light emitting apparatus according; and a control unitwhich controls an emitting state of the light emitting apparatus.

Still another embodiment of the present invention relates to a methodfor controlling a capacitor charging apparatus, including a transformerand an output capacitor charged with current flowing through a secondarycoil of the transformer, which charges the output capacitor byperforming a switching control of a switching transistor provided on apath leading to a primary coil of the transformer. This control methodcomprising: an off-signal generating step of monitoring current flowingthrough the primary coil of the transformer and generating an off signalof a predetermined level when the current rises up to a predeterminedpeak current; a first on-signal generating step of monitoring voltageacross the primary coil of the transformer and generating a first onsignal of a predetermined level when the voltage across the primary coilthereof drops down to a predetermined threshold voltage; a secondon-signal generating step of monitoring a monitoring voltagecorresponding to an output voltage appearing at the output capacitor,setting an off-time based on the monitoring voltage and generating asecond on signal of a predetermined level after the off signal of thepredetermined level has been outputted and then the off-time haselapsed; and a switching step of turning off the switching transistoraccording to the off signal and turning on the switching transistoraccording to the first and the second on signal.

According to this embodiment, the selection as to whether priority isgiven to the efficiency or the charging rate can be controlled.

When the off signal of the predetermined level is outputted, theswitching step may turn off the switching transistor, and the switchingstep may turn on the switching transistor according to the on signal,whichever first goes to the predetermined level, of either the first onsignal or the second on signal.

It is to be noted that any arbitrary combination or rearrangement of theabove-described structural components and so forth are effective as andencompassed by the present embodiments.

Moreover, this summary of the invention does not necessarily describeall necessary features so that the invention may also be sub-combinationof these described features.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of examples only, withreference to the accompanying drawings which are meant to be exemplary,not limiting and wherein like elements are numbered alike in severalFigures in which:

FIG. 1 is a block diagram showing a structure of an electronic apparatushaving a light emitting apparatus according to one embodiment of thepresent invention;

FIG. 2 is a circuit diagram showing a structure of a light emittingapparatus according to a first embodiment of the present invention.

FIG. 3 is a timing chart showing an operation of a capacitor chargingapparatus of FIG. 2;

FIG. 4 is a circuit diagram showing a structure of a light emittingapparatus according to a second embodiment of the present invention;

FIG. 5 is a circuit diagram showing a structure of a driver circuit ofFIG. 4;

FIG. 6 is a timing chart showing an operation of a capacitor chargingapparatus of FIG. 4;

FIGS. 7A to 7C are timing charts each showing a base current supplied toa switching transistor and a primary current;

FIG. 8 is a circuit diagram showing a structure of a light emittingapparatus according to a third embodiment of the present invention;

FIG. 9 is a timing chart showing a switching operation in response to afirst on signal of a capacitor charging apparatus shown in FIG. 8;

FIG. 10 is a timing chart showing a switching operation in response to asecond on signal of a capacitor charging apparatus shown in FIG. 8;

FIG. 11 is a circuit diagram showing a structure of a switching controlunit in a capacitor charging apparatus according to a fourth embodimentof the present invention;

FIG. 12 is a circuit diagram showing a modification of the switchingcontrol unit shown in FIG. 11;

FIG. 13 is a circuit diagram showing a partial structure of a capacitorcharging apparatus according to a fifth embodiment of the presentinvention;

FIG. 14 is a block diagram showing a structure of a capacitor chargingapparatus according to a sixth embodiment of the present invention;

FIG. 15 is a circuit diagram showing a modification of the capacitorcharging apparatus shown in FIG. 14; and

FIG. 16 is a circuit diagram showing a structure of a chargingcompletion detecting circuit that detects the completion of charging.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described based on preferred embodiments whichdo not intend to limit the scope of the present invention but exemplifythe invention. All of the features and the combinations thereofdescribed in the embodiments are not necessarily essential to theinvention.

First Embodiment

FIG. 1 is a block diagram showing a structure of an electronic apparatus300 provided with a light emitting apparatus 200 according to a firstembodiment of the present invention. The electronic apparatus 300, whichis a digital still camera, a digital video camera or a mobile phoneterminal provided with an image pickup function, includes a battery 310,a DSP (Digital Signal Processor) 314, an image pickup unit 316, and alight emitting apparatus 200.

The battery 310, which is, for instance, a lithium ion battery, outputsa voltage of about 3 to 4 V as a battery voltage Vbat. The DSP 314,which is a block for performing an overall control of the electronicapparatus 300, is connected to the image pickup unit 316 and the lightemitting apparatus 200. The image pickup unit 316 is an image-takingdevice such as a CCD (Charge-Coupled Device) or a CMOS sensor. The lightemitting apparatus 200 is a light source that is used as a flash at thetime of image pickup by the image pickup unit 316.

The light emitting apparatus 200 includes a capacitor charging apparatus210, a light-emitting element 212, and a trigger circuit 214. A xenontube, for instance, is suitably used as the light-emitting element 212.The capacitor charging apparatus 210 boosts a battery voltage Vbatsupplied from the battery 310 by charging an output capacitor providedat the output thereof and supplies a drive voltage of about 300 V to thelight-emitting element 212. The trigger circuit 214 is a circuit thatcontrols the light emission timing of the light emitting apparatus 200.The light-emitting element 212 emits light synchronously with the imagepickup by the image pickup unit 316.

FIG. 2 is a circuit diagram showing a structure of a light emittingapparatus 200 of FIG. 1. The light emitting apparatus 200 includes acapacitor charging apparatus 210, a light-emitting element 212 and anIGBT 214 a. A control circuit 100, a switching transistor Tr1, atransformer 10, a rectifier diode D1 and an output capacitor C1 shown inFIG. 2 correspond to the capacitor charging apparatus 210 shown inFIG. 1. The trigger circuit 214 shown in FIG. 1 corresponds to the IGBT214 a and the light emission control unit 214 b of FIG. 2.

The capacitor charging apparatus 210 generates a drive voltage(hereinafter referred to also as output voltage Vout) necessary for thelight emission by the light-emitting element 212 by supplying a chargingcurrent to the output capacitor C1. The capacitor charging apparatus 210is structured by including an output circuit 20 and a control circuit100.

The output circuit 20 includes a transformer 10, a rectifier diode D1,and an output capacitor C1. The transformer 10 is provided with aprimary coil 12 and a secondary coil 14. One end of the primary coil 12is the input terminal 202 of the capacitor charging apparatus 210, wherea battery voltage Vbat outputted from the battery 310 of FIG. 1 isapplied. The other end of the primary coil 12 is connected to theswitching terminal 102 of the control circuit 100.

One end of the secondary coil 14 of the transformer 10 is grounded tofix the potential, whereas the other end thereof is connected to ananode of the rectifier diode D1. One end of the output capacitor C1 isgrounded, and the other end thereof is connected to a cathode of therectifier diode D1. A terminal of the output capacitor C1 serves as theoutput terminal 204 of the capacitor charging apparatus 210 and thusoutputs the voltage charged in the output capacitor C1 as an outputvoltage Vout.

According to the first embodiment, the secondary coil 14 of thetransformer 10 is provided with a tap 16 (F winding). The voltage of thetap 16 (hereinafter referred to as monitoring voltage Vmoni) is inputtedto a voltage monitoring terminal 108 of the control circuit 100 via awiring 18. The control circuit 100 controls the on and off of theswitching transistor Tr1 by regarding the monitoring voltage Vmoni asthe output voltage Vout of the light emitting apparatus 200. The voltagemonitoring terminal 108 and the wiring 18 leading to the tap 16 functionas a voltage detector that monitors the voltage of the tap 16.

The control circuit 100 stores energy in the transformer 10, generates acharging current for the output capacitor C1, and boosts the voltage ofthe battery voltage Vbat by performing a switching control of the on andoff of the switching transistor Tr1. Hereinbelow, the current flowingthrough the primary coil 12 is called a primary current Ic1, and thatflowing through the secondary coil 14 a secondary current Ic2.

The control circuit 100 includes a detection resistor R1, a comparator32, an off-time setting circuit 34, a driver circuit 36, a lightemission control unit 214 b, and a charging completion detecting circuit50, in addition to the switching transistor Tr1. The control circuit 100is integrated onto a single semiconductor substrate as a function IC.

The control circuit 100 controls the on and off of the switchingtransistor Tr1 by controlling the voltage or current that is applied tothe control terminal thereof. In this first embodiment, the switchingtransistor Tr1 is a bipolar transistor. The collector of the switchingtransistor Tr1 is coupled to the primary coil 12 of the transformer 10via a switching terminal 102. The driver circuit 36 performs a switchingcontrol of the base current Ib of the switching transistor Tr1.

The detection resistor R1, the comparator 32, the off-time settingcircuit 34, the driver circuit 36, and a charging completion detectingcircuit 50, in combination, function as a switching control unit forcontrolling the switching operation of the switching transistor Tr1.This switching control unit controls the on and off of the switchingtransistor Tr1 by regarding the monitoring voltage Vmoni as the outputvoltage Vout of the capacitor charging apparatus 210.

The detection resistor R1 functions as a primary current detectioncircuit that detects the primary current Ic1 flowing through the primarycoil 12 of the transformer 10. The detection resistor R1 is provided onthe same path as the primary coil 12 where the primary current Ic1 flowsand the switching transistor Tr1 are, and one end thereof is groundedand the other end thereof is connected to the emitter of the switchingtransistor Tr1. Across the detection resistor R1 occurs a voltage drop(Vdet=Ic1×R1) in proportion to the primary current Ic1. The detectionresistor R1 outputs a detection voltage Vdet according to the primarycurrent Ic1.

To a charging current control terminal 104 of the control circuit 100, acurrent adjusting signal Vadj, which is used to specify the chargingcurrent for the output capacitor C1, is inputted from outside. Thecomparator 32 compares the detection voltage Vdet outputted from theprimary current detection circuit against the current adjusting signalVadj. The comparator 32 outputs a high-level signal when it detects thatthe detection voltage Vdet has exceeded the current adjusting signalVadj, that is, the primary current Ic1 has reached a predeterminedcurrent value determined according to the current adjusting signal Vadj(hereinafter referred to as a peak current value Ipeak). A comparisonsignal Vcmp outputted from the comparator 32 is inputted to the off-timesetting circuit 34. As will be described later, the current adjustingsignal Vadj is a signal that defines the peak value Ipeak of a chargingcurrent. The relationship between the peak current value Ipeak and thecurrent adjusting signal Vadj is expressed as Ipeak=Vadj/R1.

The off-time setting circuit 34 counts a certain off-time Toff from thepoint when a comparison signal Vcmp goes high and generates a drivesignal Vdrv of a first level (e.g., low level) for a period until thisoff-time Toff elapses.

While the drive signal Vdrv is at the first level, i.e., while theoff-time Toff is being counted by the off-time setting circuit 34, thedriver circuit 36 stops the supply of base current to the switchingtransistor Tr1, thereby turning off the switching transistor Tr1. As thedrive signal Vdrv returns to a second level (e.g., high level) after thelapse of the off-time Toff, the driver circuit 36 supplies base currentto the switching transistor Tr1, thereby turning it on again.

The switching control unit 40 outputs to the base of the switchingtransistor Tr1 a switching signal Vsw for specifying “on” to theswitching transistor Tr1 until the primary current Ic1 reaches apredefined peak current value Ipeak, and then specifying “off” to theswitching transistor Tr1 for a period of a certain off-time.

Note that the off-time Toff may be a predetermined duration of time, maybe set according to the output voltage Vout, may be set according to thecondition of the primary side or the secondary side of the transformer10, or may be set according to the monitoring voltage Vmoni as will beexplained later.

Inputted to the driver circuit 36 in the switching control unit 40 isthe current adjusting signal Vadj that specifies the charging current.In response to the current adjusting signal Vadj, the driver circuit 36adjusts the switching signal to be outputted to the base of theswitching transistor Tr1. More specifically, the switching control unit40 adjusts the current value of the base current Ib to be outputted as aswitching signal to the base of the switching transistor Tr1, accordingto the current adjusting signal Vadj.

The charging completion detecting circuit 50, which is a comparator,detects the completion of a charging by regarding the monitoring voltageVmoni appearing at the tap 16 provided on the secondary coil 14 of thetransformer 10 as the output voltage Vout appearing at the outputcapacitor C1 and comparing it against a predetermined threshold voltageVth. The threshold voltage Vth is set to a voltage sufficient for lightemission by the light-emitting element 212, for example, about 300 V.Upon detecting the completion of charging, the charging completiondetecting circuit 50 sets a flag FULL to indicate the completion ofcharging. Now with the completion of charging detected by the chargingcompletion detecting circuit 50, the switching control unit 40 stops theswitching of the switching transistor Tr1.

Further, according to the first embodiment, the monitoring voltage Vmoniinputted to the voltage monitoring terminal 108 is inputted to theoff-time setting circuit 34 in the switching control unit 40. Theoff-time setting circuit 34 operates in a manner such that the off-timeToff, during which the switching transistor Tr1 is turned off, changesaccording to the monitoring voltage Vmoni.

For example, the off-time setting circuit 34 may set the off-time Toffin such a manner that the higher the monitoring voltage Vmoni is, thatis, the higher the output voltage Vout is, the shorter the off-time Toffmay be. If the off-time setting circuit 34 is to be structured by a CRtime constant circuit capable of charging and discharging the capacitor,then the off-time Toff can be adjusted by changing the charging currentor the discharging current according to the monitoring voltage Vmoni.

The light emission control unit 214 b generates a light emission controlsignal Vcnt and controls the base voltage of the IGBT 214 a. When thelight emission control signal Vent goes to a high level with a chargingof the output capacitor C1 completed and sufficient drive voltage Voutgenerated, the IGBT 214 a turns on and the light-emitting element 212emits light.

Now a description will be given of an operation of a light emittingapparatus 200 configured as described above. FIG. 3 is a timing chartshowing an operation of a capacitor charging apparatus 210 according tothe first embodiment. Note that the vertical axis and the horizontalaxis of FIG. 3 are enlarged or reduced as appropriate for ease ofunderstanding and also that the waveforms shown are simplified foreasier understanding. At time t0, a switching signal Vsw goes to a highlevel, that is, the switching transistor Tr1 turns on with a basecurrent Ib supplied thereto. With the switching transistor Tr1 turnedon, a primary current Tel flowing through the primary coil of thetransformer 10 increases gradually until time t1 when Vdet>Vadj.

With Vdet>Vadj, a comparison signal Vcmp, which is outputted from thecomparator 32, switches from low level to high level. The off-timesetting circuit 34 sets the drive signal Vdrv to a first level (lowlevel) for an off-time Toff after the comparison signal Vcmp has reacheda high level. For the duration when the drive signal Vdrv is at a lowlevel, the driver circuit 36 turns off the switching transistor Tr1 bystopping the supply of base current Ib thereto. With the switchingtransistor Tr1 turned off, the output capacitor C1 is charged with asecondary current Ic2 flowing through the secondary coil 14 of thetransformer 10.

At time t2, after the passage of an off-time Toff from time t1, thedrive signal Vdrv switches to a high level. Now, with the drive signalVdrv at high level, the driver circuit 36 supplies base current Ib tothe switching transistor Tr1. By repeating the cycle of operation fromtime t0 to time t2, the control circuit 100 charges the output capacitorC1 and raises the output voltage Vout.

If it is so arranged that the off-time Toff to be set by the off-timesetting circuit 34 is dependent on the monitoring voltage Vmoni, thenthe off-time Toff will grow shorter gradually with the rise in themonitoring voltage Vmoni, namely, the rise in the output voltage Vout.Consequently, when the output voltage Vout is low immediately after thestart of charging, the energy stored in the transformer is usedefficiently, and as the output voltage Vout goes higher, the chargingrate can be accelerated. Thus, it is possible to balance the efficiencyand the charging rate.

At time t3, when the monitoring voltage Vmoni reaches the thresholdvoltage Vth, a flag FULL indicating the completion of a charging is setby the charging completion detecting circuit 50, which enables a lightemission by the light-emitting element 212. When the monitoring voltageVmoni rises to a desired voltage level, the light emission control unit214 b switches the light emission control signal Vent to a high level insynchronism with an image pickup by the image pickup unit 316 as shownin FIG. 1. As a result, the IGBT 214 a turns on, and the xenon lamp,which is the light-emitting element 212, emits light as a flash.

With the capacitor charging apparatus 210 according to the firstembodiment, it is possible to detect with accuracy the voltage relativeto the output voltage Vout appearing at the output capacitor C1 bymonitoring the potential at a tap (F winding) provided on the secondarycoil 14 of the transformer 10. Also, location of the tap 16 on thegrounding side, namely the lower-voltage side, of the secondary coil 14makes it unnecessary to use resistance voltage division, thus avoidingany increase in the number of circuit components used. The controlcircuit 100 can optimally perform switching control of the switchingtransistor Tr1 using the monitoring voltage Vmoni corresponding to theoutput voltage Vout.

For example, according to the first embodiment, the charging completiondetecting circuit 50 carries out the detection of a charging state basedon the monitoring voltage Vmoni. Compared with the direct monitoring ofthe output voltage Vout appearing at the output capacitor C1, thisarrangement makes it possible to stop the switching by detecting a fullcharging state with high accuracy. This will also solve the problems ofinability to obtain the voltage required to drive the load orovercharging of the output capacitor C1.

Also, according to the first embodiment, the off-time setting circuit 34sets the off-time Toff based on the monitoring voltage Vmoni. As aresult, the output voltage can be reflected in the switching operationwith accuracy.

The above embodiment is merely exemplary and it is understood by thoseskilled in the art that various modifications to the combination of eachcomponent and process thereof are possible and such modifications arealso within the scope of the present invention.

In the present embodiment, a description has been given of a case wherea bipolar transistor is used as the switching transistor Tr1. However, aMOSFET may be used instead. Also, in the present embodiment, adescription has been given of a case where the timing of on and off ofthe switching transistor Tr1 is determined based on the primary currentIc1. However, the control method is not limited thereto, and othercontrol methods may be employed.

Further, in the present embodiment, a description has been given of acase where a capacitor charging apparatus 210 drives a light-emittingelement 212. However, the application is not limited thereto, and thecapacitor charging apparatus 210 may also be used to drive a variety ofother load circuits that require high voltage.

Also, the setting of logical values of high level and low leveldescribed in the present embodiment is only one example. The setting canbe changed freely by inverting them as appropriate by an inverter or thelike.

Second Embodiment

FIG. 4 is a circuit diagram showing a structure of a light emittingapparatus 200 a according to a second embodiment of the presentinvention. Similar to the light emitting apparatus 200 according to thefirst embodiment, the light emitting apparatus 200 a as shown in FIG. 4is suitably used for the electronic apparatus as shown FIG. 1.

The electronic apparatus 300 of FIG. 1, which is a digital still camera,a digital video camera or a mobile phone terminal provided with an imagepickup function, includes a battery 310, a DSP (Digital SignalProcessor) 314, an image pickup unit 316, and a light emitting apparatus200 a.

The battery 310, which is, for instance, a lithium ion battery, outputsa voltage of about 3 to 4 V as a battery voltage Vbat. The DSP 314,which is a block for performing an overall control of the electronicapparatus 300, is connected to the image pickup unit 316 and the lightemitting apparatus 200 a. The image pickup unit 316 is an image-takingdevice such as a CCD (Charge-Coupled Device) or a CMOS sensor. The lightemitting apparatus 200 a is a light source that is used as a flash atthe time of image pickup by the image pickup unit 316.

The light emitting apparatus 200 a includes a capacitor chargingapparatus 210, a light-emitting element 212, and a trigger circuit 214.A xenon tube or the like is suitably used as the light-emitting element212. The capacitor charging apparatus 210 boosts a battery voltage Vbatsupplied from the battery 310 by charging an output capacitor providedat the output thereof and supplies a drive voltage of about 300 V to thelight-emitting element 212. The trigger circuit 214 is a circuit thatcontrols the light emission timing of the light emitting apparatus 200a. The light-emitting element 212 emits light synchronously with theimage pickup by the image pickup unit 316.

Referring back to FIG. 4, a structure of the light emitting apparatus200 a will now be described. The light emitting apparatus 200 a includesa capacitor charging apparatus 210, a light-emitting element 212, and anIGBT 214 a. A control circuit 100, a switching transistor Tr1, atransformer 10, a rectifier diode D1 and an output capacitor C1 shown inFIG. 4 correspond to the capacitor charging apparatus 210 shown inFIG. 1. The trigger circuit 214 shown in FIG. 1 corresponds to the IGBT214 a and the light emission control unit 214 b of FIG. 4.

The capacitor charging apparatus 210 generates a drive voltage(hereinafter referred to also as output voltage Vout) necessary for thelight emission by the light-emitting element 212 by supplying a chargingcurrent to the output capacitor C1. The capacitor charging apparatus 210is structured by including an output circuit 20 and a control circuit100.

The output circuit 20 includes a transformer 10, a rectifier diode D1,and an output capacitor C1. The transformer 10 is provided with aprimary coil 12 and a secondary coil 14. One end of the primary coil 12is the input terminal 202 of the capacitor charging apparatus 210, wherethe battery voltage Vbat outputted from the battery 310 of FIG. 1 isapplied. The other end of the primary coil 12 is connected to theswitching terminal 102 of the control circuit 100.

One end of the secondary coil 14 of the transformer 10 is grounded tofix the potential, whereas the other end thereof is connected to ananode of the rectifier diode D1. One end of the output capacitor C1 isgrounded, and the other end thereof is connected to a cathode of therectifier diode D1. A terminal of the output capacitor C1 serves as theoutput terminal 204 of the capacitor charging apparatus 210 and thusoutputs the voltage charged in the output capacitor C1 as an outputvoltage Vout.

The control circuit 100 stores energy in the transformer 10, generates acharging current for the output capacitor C1, and boosts the voltage ofthe battery voltage Vbat by performing a switching control of the on andoff of the switching transistor Tr1. Hereinbelow, the current flowingthrough the primary coil 12 is called a primary current Ic1, and thatflowing through the secondary coil 14 a secondary current Ic2.

The control circuit 100 includes a detection resistor R1, a comparator32, an off-time setting circuit 34, a driver circuit 36 and a lightemission control unit 214 b, in addition to the switching transistorTr1. The control circuit 100 is integrated onto a single semiconductorsubstrate as a function IC.

The control circuit 100 controls the on and off of the switchingtransistor Tr1 by controlling the voltage or current that is applied tothe control terminal thereof. In this second embodiment, the switchingtransistor Tr1 is a bipolar transistor. The collector of the switchingtransistor Tr1 is coupled to the primary coil 12 of the transformer 10via the switching terminal 102. The driver circuit 36 performs aswitching control of the base current Ib of the switching transistorTr1.

The detection resistor R1 functions as a primary current detectioncircuit 30 that detects the primary current Ic1 flowing through theprimary coil 12 of the transformer 10. The detection resistor R1 isprovided on the same path as the primary coil 12 where the primarycurrent Ic1 flows and the switching transistor Tr1 are, and one endthereof is grounded and the other end thereof is connected to theemitter of the switching transistor Tr1. Across the detection resistorR1 occurs a voltage drop (Vdet=Ic1×R1) in proportion to the primarycurrent Ic1. The detection resistor R1 outputs a detection voltage Vdetaccording to the primary current Ic1.

To a charging current control terminal 104 of the control circuit 100, acurrent adjusting signal Vadj, which is used to specify the chargingcurrent for the output capacitor C1, is inputted from outside. Thecomparator 32 compares the detection voltage Vdet outputted from theprimary current detection circuit 30 against the current adjustingsignal Vadj. The comparator 32 outputs a high-level signal when itdetects that the detection voltage Vdet has exceeded the currentadjusting signal Vadj, that is, the primary current Ic1 has reached apredetermined peak current value determined according to the currentadjusting signal Vadj (hereinafter referred to as a peak current valueIpeak). A comparison signal Vcmp outputted from the comparator 32 isinputted to the off-time setting circuit 34. As will be described later,the current adjusting signal Vadj is a signal that defines the peakvalue Ipeak of a charging current. The relationship between the peakcurrent value Ipeak and the current adjusting signal Vadj is expressedas Ipeak=Vadj/R1.

The off-time setting circuit 34 generates a derive signal Vdrv of afirst level (e.g., low level) for a period when a comparison signal Vcmpgoes high until a certain off-time Toff elapses. While the drive signalVdrv is in the first level, the driver circuit 36 stops the supply ofbase current to the switching transistor Tr1 and thereby turns off theswitching transistor Tr1. As the drive signal Vdrv returns to a secondlevel (e.g., high level) after the lapse of the off-time Toff, thedriver circuit 36 supplies base current to the switching transistor Tr1and turns it on again.

That is, according to the second embodiment, the comparator 32, theoff-time setting circuit 34 and the driver circuit 36 monitor at leastthe primary current Tel detected by the primary current detectioncircuit 30 and function as a switching control unit 40 that controls theon and off of the switching transistor Tr1. This switching control unit40 outputs to the base of the switching transistor Tr1 a switchingsignal Vsw specifying “on” to the switching transistor for a perioduntil the primary current Tel has reached a predetermined peak currentvalue Ipeak and then specifying “off” to the switching transistor Tr1for a period of a certain off-time.

Note that the off-time Toff may be a predetermined duration of time, maybe set according to the output voltage Vout, or may be set according tothe condition of the primary side or the secondary side of thetransformer 10.

Inputted to the driver circuit 36 in the switching control unit 40 isthe current adjusting signal Vadj that specifies the charging current.In response to the current adjusting signal Vadj, the driver circuit 36adjusts the switching signal to be outputted to the base of theswitching transistor Tr1. More specifically, the switching control unit40 adjusts the current value of the base current Ib to be outputted as aswitching signal to the base of the switching transistor Tr1, accordingto the current adjusting signal Vadj.

FIG. 5 is a circuit diagram showing a structure of the driver circuit 36of FIG. 4. The driver circuit 36 includes a current generation unit 38.The current generation unit 38 includes, for example, a VI conversioncircuit 38 a and a current amplifier circuit 38 b. The VI conversioncircuit 38 a receives a current adjusting signal Vadj and generates acurrent Idrv1 according to the voltage value of the current adjustingsignal Vadj. The VI conversion circuit 38 a generates the drive currentIdrv in such a manner that the larger the current adjusting signal Vadjis, the larger the drive current Idrv1 is. Preferably, the drive currentIdrv1 may be proportional to the current adjusting signal Vadj.

The current Idrv1 is amplified by the current amplifier circuit 38 bconfigured by a current mirror circuit and the like. For a period whenthe drive signal Vdrv outputted from the off-time setting circuit 34 isin a second level, the switch circuit 39 outputs the drive current Idrv2as the drive current Ib of the switching transistor Tr1, whereas for aperiod when the derive signal Vdrv is in a first level, it stops theoutput of the drive current Idrv2.

Refer back to FIG. 4. The charging completion detecting circuit 50monitors either directly or indirectly the output voltage Vout appearingat the output capacitor C1. “Directly monitoring” includes a case wherethe potential of the output terminal 204 is subjected to the resistancevoltage division. “Indirectly monitoring” includes a case where thevoltage applied to the primary coil 12 or the secondary coil 14 of thetransformer 10 is monitored.

The charging completion detecting circuit 50, which is a comparator,detects the completion of a charging by comparing an output voltageVout′ corresponding to the output voltage Vout against a predeterminedthreshold voltage Vth. The threshold voltage Vth is set to a voltagesufficient for light emission by the light-emitting element 212, forexample, about 300 V. Upon detecting the completion of charging, thecharging completion detecting circuit 50 sets a flag FULL to indicatethe completion of charging. Now with the completion of charging detectedby the charging completion detecting circuit 50, the switching controlunit 40 stops the switching of the switching transistor Tr1.

The light emission control unit 214 b generates a light emission controlsignal Vcnt and controls the base voltage of the IGBT 214 a. When thelight emission control signal Vcnt goes to a high level with a chargingof the output capacitor C1 completed and sufficient drive voltage Voutgenerated, the IGBT 214 a turns on and the light-emitting element 212emits light.

Now a description will be given of an operation of a light emittingapparatus 200 a configured as described above. FIG. 6 is a timing chartshowing an operation of a capacitor charging apparatus 210 according tothe second embodiment. Note that the vertical axis and the horizontalaxis of FIG. 6 are enlarged or reduced as appropriate for ease ofunderstanding and also that the waveforms shown are simplified foreasier understanding.

At time t0, a switching signal Vsw goes to a high level, that is, theswitching transistor Tr1 turns on with a base current Ib suppliedthereto. With the switching transistor Tr1 turned on, a primary currentIc1 flowing through the primary coil of the transformer 10 increasesgradually until time t1 when Vdet>Vadj.

With Vdet>Vadj, a comparison signal Vcmp, which is outputted from thecomparator 32, switches from low level to high level. The off-timesetting circuit 34 sets the drive signal Vdrv to a first level (lowlevel) for an off-time Toff after the comparison signal Vcmp has becomea high level. For the duration when the drive signal Vdrv is at a lowlevel, the driver circuit 36 turns off the switching transistor Tr1 bystopping the supply of base current Ib thereto. With the switchingtransistor Tr1 turned off, the output capacitor C1 is charged with asecondary current Ic2 flowing through the secondary coil 14 of thetransformer 10.

At time t2, after the passage of an off-time Toff from time t1, thedrive signal Vdrv switches to a high level. Now, with the drive signalVdrv at high level, the driver circuit 36 supplies base current Ib tothe switching transistor Tr1. By repeating the cycle of operation fromtime t0 to time t2, the control circuit 100 charges the output capacitorC1 and raises the output voltage Vout.

At time t3, when the output voltage Vout reaches the threshold voltageVth, a flag FULL indicating the completion of a charging is set, therebyenabling a light emission by the light-emitting element 212. When theoutput voltage Vout rises to a desired voltage level, the light emissioncontrol unit 214 b switches the light emission control signal Vent to ahigh level in synchronism with an image pickup by the image pickup unit316 as shown in FIG. 1. As a result, the IGBT 214 a turns on, and thexenon lamp, which is the light-emitting element 212, emits light as aflash.

FIGS. 7A to 7C are timing charts each showing a base current Ib suppliedto a switching transistor Tr1 and a primary current Ic1. FIG. 7Aillustrates a case where the current adjusting signal Vadj is small andtherefore the charging current is set to a small value, whereas FIG. 7Cillustrates a case where the current adjusting signal Vadj is set to amaximum value and therefore the charging current is also maximum. FIG.7B illustrates an intermediate case of the charging current in betweenthe above two cases. The driver circuit 36 sets the level of the basecurrent Ib in accordance with the current adjusting signal Vadj, namelythe peak value Ipeak of the primary current Ic1. As a result, the basecurrent Ib in the case of FIG. 7B is larger than that in the case ofFIG. 7A.

In the capacitor charging apparatus 210 according to the secondembodiment, the charging current is set in response to the peak currentIpeak of the primary current Ic1. It is required that the base currentin response to a collector current (i.e., primary current Ic1) besupplied in order to stably drive a bipolar transistor. Thus, in a casewhen the level of the base current Ib is to be fixed to a constantvalue, it is required that a base current associated with the maximumvalue Imax of an assumed peak current Ipeak be set beforehand. In FIGS.7A to 7C, such a base current is shown as a dotted line. In this case,when a charging current lower than the maximum value Imax is set, a basecurrent larger than the normally required base current is supplied tothe switching transistor Tr1 and therefore a wasteful power will beconsumed.

In contrast to this, the capacitor charging apparatus 210 according tothe second embodiment adjusts the base current Ib in response to thecharging current, namely, the primary current Ic1 flowing through theswitching transistor Tr1 as a collector current. Accordingly, in a casewhen the primary current Ic1 is small, the base current Ib is also setto a small value, thus preventing the wasteful consumption of current.

The above-described embodiments are merely exemplary, and it isunderstood by those skilled in the art that various modifications to thecombination of each component and process thereof are possible and suchmodifications are also within the scope of the present invention.

In the above embodiments a description has been given of a case where abipolar transistor is used as the switching transistor Tr1, but a MOSFETmay be used instead. In such a case, too, the same suitable drive as inthe case of the bipolar can be realized. That is, if the level ofcharging current is large, a gate drive current may be increased or theamplitude of gate voltage may be set to a large value.

In the above embodiments a description was given of a case where thecapacitor charging apparatus 210 drives the light emitting element 212,but this should not be considered as liming, and it can drive a varietyof other load circuits requiring high voltage.

In the above embodiments, the setting of logical values of high leveland low level is one example among many and it may be arbitrarilychanged as appropriate by inversion by an inverter or the like.

Third Embodiment

FIG. 1 is a block diagram showing a structure of an electronic apparatus300 outfitted with a light emitting apparatus 200 c according to a thirdembodiment of the present invention. An electronic apparatus 300, whichis a digital still camera, a digital video camera or a mobile phoneterminal provided with an image pickup function, includes a battery 310,a DSP (Digital Signal Processor) 314, an image pickup unit 316, and alight emitting apparatus 200 c.

The battery 310, which is, for instance, a lithium ion battery, outputsa voltage of about 3 to 4 V as a battery voltage Vbat. The DSP 314,which is a block for performing an overall control of the electronicapparatus 300, is connected to the image pickup unit 316 and the lightemitting apparatus 200 c. The image pickup unit 316 is an image-takingdevice such as a CCD (Charge-Coupled Device) or a CMOS sensor. The lightemitting apparatus 200 c is a light source that is used as a flash atthe time of image pickup by the image pickup unit 316.

The light emitting apparatus 200 c includes a capacitor chargingapparatus 210, a light-emitting element 212, and a trigger circuit 214.A xenon tube or the like is suitably used as the light-emitting element212. The capacitor charging apparatus 210 boosts a battery voltage Vbatsupplied from the battery 310 by charging an output capacitor providedat the output thereof and supplies a drive voltage of about 300 V to thelight-emitting element 212. The trigger circuit 214 is a circuit thatcontrols the light emission timing of the light emitting apparatus 200c. The light-emitting element 212 emits light synchronously with theimage pickup by the image pickup unit 316.

FIG. 8 is a circuit diagram showing a structure of the light emittingapparatus 200 c according to the third embodiment. The light emittingapparatus 200 c includes a capacitor charging apparatus 210, alight-emitting element 212, and an IGBT 214 a. A control circuit 100 c,a switching transistor Tr1, a transformer 10, a rectifier diode D1 andan output capacitor C1 shown in FIG. 8 correspond to the capacitorcharging apparatus 210 shown in FIG. 1. The trigger circuit 214 shown inFIG. 1 corresponds to the IGBT 214 a and the light emission control unit214 b of FIG. 8.

The capacitor charging apparatus 210 generates a drive voltage(hereinafter referred to also as output voltage Vout) necessary for thelight emission by the light-emitting element 212 by supplying a chargingcurrent to the output capacitor C1. The capacitor charging apparatus 210is structured by including an output circuit 20 and a control circuit100 c.

The output circuit 20 includes a transformer 10, a rectifier diode D1,and an output capacitor C1. The transformer 10 is provided with aprimary coil 12 and a secondary coil 14. One end of the primary coil 12is the input terminal 202 of the capacitor charging apparatus 210, wherethe battery voltage Vbat outputted from the battery 310 of FIG. 1 isapplied. The other end of the primary coil 12 is connected to theswitching terminal 102 of the control circuit 100 c.

One end of the secondary coil 14 of the transformer 10 is grounded tofix the potential, whereas the other end thereof is connected to ananode of the rectifier diode D1. One end of the output capacitor C1 isgrounded, and the other end thereof is connected to a cathode of therectifier diode D1. A terminal of the output capacitor C1 serves as anoutput terminal 204 of the capacitor charging apparatus 210 and thusoutputs the voltage charged in the output capacitor C1 as an outputvoltage Vout.

The control circuit 100 c stores energy in the transformer 10, generatesa charging current for the output capacitor C1, and boosts the voltageof the battery voltage Vbat by performing a switching control of the onand off of the switching transistor Tr1. Hereinbelow, the currentflowing through the primary coil 12 is called a primary current Ic1, andthat flowing through the secondary coil 14 a secondary current Ic2.

The control circuit 100 c includes an off-signal generator 80, a firston-signal generator 90, a second on-signal generator 96, a switchingcontrol unit 60 and a charging completion detecting circuit 70, inaddition to the switching transistor Tr1. The control circuit 100 c isintegrated onto a single semiconductor substrate as a function IC.

The off-signal generator 80 monitors the primary current Ic1 flowingthrough the primary coil 12 of the transformer 10, and outputs an offsignal Soff when the primary current Ic1 increases up to and reaches apredetermined peak current Ipeak. The off-signal generator 80 includes afirst resistor R1 and a first comparator 82. The detection resistor R1is provided on the same path as the primary coil 12 where the primarycurrent Ic1 flows and the switching transistor Tr1 are, and one endthereof is grounded and the other end thereof is connected to theemitter of the switching transistor Tr1. Across the detection resistorR1 occurs a voltage drop (Vdet=Ic1×R1) in proportion to the primarycurrent Ic1. The detection resistor R1 outputs a detection voltage Vdetaccording to the primary current Ic1.

To a charging current control terminal 104 of the control circuit 100 c,a current adjusting signal Vadj, which is used to specify the chargingcurrent for the output capacitor C1, is inputted from outside. The firstcomparator 82 compares the detection voltage Vdet outputted from theprimary current detection circuit against the current adjusting signalVadj. The first comparator 82 outputs a high-level off signal Soff whenit detects that the detection voltage Vdet has exceeded the currentadjusting signal Vadj, that is, the primary current Ic1 has reached apredetermined current value determined according to the currentadjusting signal Vadj (hereinafter referred to as peak current valueIpeak). The off signal Soff outputted from the first comparator 82 isinputted to the switching control unit 60 and the second on-signalgenerator 96. The relationship between the peak current value Ipeak andthe current adjusting signal Vadj is expressed as Ipeak=Vadj/R1.

The switching terminal 102 of the control circuit 100 c is connected toone end of the primary coil 12. The battery voltage Vbat outputted fromthe battery 310 is inputted to an input voltage terminal 110. A secondcomparator 94 shifts a voltage appearing at the switching terminal 102to the high-potential side by a first threshold voltage Vth1 generatedfrom a voltage supply 92 so as to compare it with the battery voltageVbat inputted to the input terminal 110. If ΔV<Vth1, a high-level signal(hereinafter referred to as first on signal Son1) will be outputted fromthe second comparator 94. That is, the first on-signal generator 90monitors the voltage ΔV across the primary coil 12 of the transformer10; and if the voltage ΔV across the primary coil 12 drops down to thefirst threshold voltage Vth1, the first on-signal generator 90 willoutput a high-level first on signal Son1. The first on signal Son1 isinputted to the switching control unit 60.

The monitoring voltage Vout′ corresponding to the output voltage Voutappearing at the output capacitor C1 is inputted to a voltage monitoringterminal 108 of the control circuit 100 c. The monitoring voltage Vout′is a voltage obtained when the output voltage Vout is voltage-divided bya resistor R2 and a resistor R3. The monitoring voltage Vout′ isinputted to the second on-signal generator 96.

The second on-signal generator 96 monitors the monitoring voltage Vout′and then sets an off-time Toff based on this monitoring voltage Vout′.When the off signal Soff of high level is outputted from the off-signalgenerator 80 and then the off-time thus set has elapsed, the secondon-signal generator 96 outputs the second on signal Son2 of high level.

For example, the second on-signal generator 96 sets the off-time Toff ina manner that the larger the monitoring voltage Vout′ is, namely, thelarger the output voltage Vout is, the shorter the off-time Toff willbecome. For example, the second on-signal generator 96 can be structuredby a CR time-constant circuit which charges/discharges a capacitor. Insuch a case, the off-time Toff can be suitably adjusted by varying thecharging or discharging current according to the monitoring voltageVout′. Also, the second on-signal generator 96 may be constituted by adigital timer.

The switching control unit 60 receives the off signal Soff outputtedfrom the off-signal generator 80, and the first on signal Son1 and thesecond on signal Son2 outputted from the first on-signal generator 90and the second on-signal generator 96, respectively. The switchingcontrol unit 60 turns off the switching transistor Tr1 in response tothe off signal Soff and turns on the switching transistor Tr1 inresponse to the first on signal Son1 and the second on signal Son2.

In the third embodiment, the switching control unit 60 includes aflip-flop 62 and a driver circuit 64. By a switch SW1, either one of thefirst on signal Son1 and the second on signal Son2 is outputted to theflip-flop 62. The switch SW1 may be controlled by a user. The off signalSoff is inputted to a reset terminal of the flip-flop 62. That is, whena high level is inputted to a set terminal of the flip-flop 62 by eitherthe first on signal Son1 or the second on signal Son2, an output signalSq of the flip-flop 62 goes to a high level. When a high level isinputted to the reset terminal of the flip-flop 62, the output signal Sqgoes to a low level.

The driver circuit 64 outputs a switching signal Vsw in response to theoutput signal Sq of the flip-flop 62, to the base of the switchingtransistor Tr1. The driver circuit 64 turns on the switching transistorTr1 when the output signal Sq of the flip-flop 62 is in a high level,whereas the driver circuit 64 turns off the switching transistor Tr1when the output signal Sq thereof is in a low level.

The charging completion detecting circuit 70, which is a comparator,detects the completion of a charging by comparing a monitoring voltageVout′ against a predetermined threshold voltage Vth3. The thresholdvoltage Vth3 is set to a voltage sufficient for light emission by thelight-emitting element 212, for example, about 300 V. When themonitoring voltage Vout′ exceeds the threshold voltage Vth3, thecharging completion detecting circuit 70 sets a flag FULL to indicatethe completion of charging. Now with the completion of charging detectedby the charging completion detecting circuit 70, the switching controlunit 60 stops the switching of the switching transistor Tr1.

The light emission control unit 214 b generates a light emission controlsignal Vcnt and controls the base voltage of the IGBT 214 a. When thelight emission control signal Vcnt goes to a high level with a chargingof the output capacitor C1 completed and sufficient drive voltage Voutgenerated, the IGBT 214 a turns on and the light-emitting element 212emits light.

Now a description will be given of an operation of the light emittingapparatus 200 c configured as described above. FIGS. 9 and 10 are each atiming chart showing an operation of a capacitor charging apparatus 210according to the third embodiment. FIG. 9 shows a switching operation inresponse to the first on signal Son1, whereas FIG. 10 shows a switchingoperation in response to the second on signal Son2. Note that thevertical axis and the horizontal axis of FIG. 9 and FIG. 10 are enlargedor reduced as appropriate for ease of understanding and also that thewaveforms shown therein are simplified for easier understanding.

With reference to FIG. 9, a description is first given of a chargingoperation according to the off signal Soff and the first on signal Son1.At time t0, a switching signal Vsw goes to a high level and thereforethe switching transistor Tr1 turns on. At this time, the output signalSq of the flip-flop is in a high level.

With the switching transistor Tr1 turned on, a primary current Ic1flowing through the primary coil 12 of the transformer 10 graduallyincreases with time and this increase in the primary current Ic1 causesthe detection voltage Vdet to rise.

When Vdet>Vadj at time t1, namely when the primary current Ic1 hasreached the peak current value Ipeak, the off signal Soff outputted fromthe first comparator 82 goes to a high level. Then the flip-flop 62 isreset and, as a result, the output signal Sq transits to a low level. Asthe output signal Sq goes to the low level, the switching signal Vswgoes also to a low level, thereby turning off the switching transistorTr1.

For a period of time t0 to t1 during which the switching transistor Tr1is on, the voltage across the primary coil 12 is ΔV≈Vbat−Vsat. Here,Vsat is the sum voltage of an emitter-collector voltage of the switchingtransistor Tr1 and the detection voltage Vdet.

As the switching transistor Tr1 turns off at time t1, the energy storedin the transformer 10 is discharged as a secondary current Ic2. Thesecondary current Ic2 flows into the output capacitor C1 as a chargingcurrent and the output voltage Vout rises. When the energy stored in thetransformer 10 has been completely expended at time t2, the outputvoltage Vout stops rising.

During a period of time t1 to t2, the voltage ΔV across the primary coil12 is ΔV≈Vout/n. Here, n is a turns ratio of the primary coil 12 and thesecondary coil 14 of the transformer 10. When the energy stored in thetransformer 10 has been completely discharged at time t2, the voltage ΔVacross the primary coil 12 damped-oscillates due to the LC oscillation.As the voltage ΔV across the primary coil 12 becomes less than the firstthreshold voltage Vth1 at time t3, the second comparator 94 outputs thefirst on signal Son1 of high level. This first on signal Son1 sets theflip-flop 62, so that the output signal Sq transits to a high level andthe switching transistor Tr1 turns on again.

By repeating the cycle of operation from t1 to t3, the control circuit100 c charges the output capacitor C1. This charging operation raisesthe output voltage Vout. And when the monitoring voltage Vout′ hasreached the threshold voltage Vth3 at time t4, a flag FULL indicatingthe completion of charging is set by the charging completion detectingcircuit 70, which enables a light emission by the light-emitting element212. After the charging has been completed, the light emission controlunit 214 b switches the light emission control signal Vcnt to a highlevel in synchronism with an image pickup by the image pickup unit 316as shown in FIG. 1. As a result, the IGBT 214 a turns on, and the xenonlamp, which is the light-emitting element 212, emits light as a flash.

Next, with reference to FIG. 10, a description is given of a chargingoperation according to the off signal Soff and the second on signalSon2. At time t0, the output signal Sq of the flip-flop 62 goes to ahigh level, thereby turning on the switching transistor Tr1.

With the switching transistor Tr1 turned on, the primary current Telflowing through the primary coil 12 of the transformer 10 graduallyincreases with time, which in turn raises the detection voltage Vdet.

With Vdet>Vadj at time t1, the off signal Soff outputted from the firstcomparator 82 goes to a high level, the flip-flop 62 is reset and theoutput signal Sq thereof transits to a low level. As the output signalSq goes to the low level, the switching signal Vsw also goes to a lowlevel, thus turning off the switching transistor Tr1.

As the switching transistor Tr1 turns off at time t1, the energy storedin the transformer 10 is discharged as a secondary current Ic2. Thesecondary current Ic2 flows into the output capacitor C1 as a chargingcurrent and the output voltage Vout rises.

The second on-signal generator 96 outputs a second on signal Son2 whichgoes to a high level from time t1 when a high-level off signal Soff isoutputted until time t2 after the passage of an off-time Toff setaccording to the monitoring voltage Vout′. The flip-flop 62 is set bythis second on signal Son2, then the output signal Sq shifts to a highlevel and the switching transistor Tr1 turns on again.

By repeating the cycle of operation from t0 to t2, the control circuit100 c charges the output capacitor C1 and raises the output voltageVout. When the monitoring voltage Vout′ has reached the thresholdvoltage Vth3 at time t3, a flag FULL indicating the completion ofcharging is set by the charging completion detecting circuit 70, whichenables a light emission by the light-emitting element 212.

In the control performed based on the first on signal Son1 of thecapacitor charging apparatus 210 according to the third embodiment, theswitching transistor Tr1 is turned on after the energy stored in thetransformer 10 has been completely discharged. Hence, the highefficiency can be realized. Also, by performing the control based on thesecond on signal Son2, the energy can be used efficiently, immediatelyafter a start of charging, because of a longer off-time. Hence, as theoutput voltage Vout increases, the charge rate can be accelerate with ashorter off-time.

Further, by employing the capacitor charging apparatus 210 according tothe third embodiment, the control can be switched between the controlbased on the first on signal Son1 as shown in FIG. 9 and the controlbased on the second on signal Son2 as shown in FIG. 10. As a result,either a circuit operation where the efficiency is given priority or acircuit operation where the charge rate is given priority can beselected according to its application or the like.

Fourth Embodiment

FIG. 11 is a circuit diagram showing a structure of a switching controlunit 60 a in a capacitor charging apparatus 210 according to a fourthembodiment of the present invention. In the third embodiment, adescription has been given of a case where the timing at which theswitching transistor Tr1 turns on is determined according to either oneof the first on signal Son1 and the second on signal Son2 selected bythe switch SW1. In contrast to this, in this fourth embodiment theswitching transistor Tr1 turns on according to the on signal, whicheverfirst goes to a high level, of either the first on signal Son1 or thesecond on signal Son2.

The switching control unit 60 a according to the fourth embodimentincludes an OR gate 66 in place of the switch SW1 of FIG. 8. Theconnection state of a flip-flop 62 and a driver circuit 64 is the sameas that shown in FIG. 8. A first on signal Son1 and a second on signalSon2 are inputted to the OR gate 66. In such a case, an output signalSon of the OR gate 66 is a logical sum of the first on signal Son1 andthe second on signal Son2. Thus, at the point when either one of thefirst on signal Son1 and the second on signal Son2 goes to a high level,the output signal Son goes to a high level, too.

The output signal Son of the OR gate 66 is inputted to a set terminal ofthe flip-flop 62. Thus, at the timing when either one of the first onsignal Son1 and the second on signal Son2 whichever first goes to a highlevel, the flip-flop 62 is rest. The reset operation of the flip-flop 62is the same as that described in the third embodiment.

According to the capacitor charging apparatus 210 of the fourthembodiment, the switching transistor Tr1 turns on in response to eitherone of first on signal Son1 and the second on signal Son2 whicheverfirst goes to a high level. Hence, even in the case when the timing atwhich the first on signal Son1 goes to a high level is delayedexcessively or the timing at which the second on signal Son2 goes to ahigh level is delayed excessively, that the off-time of the switchingtransistor Tr1, namely, the discharge time of energy stored in thetransformer 10 becomes longer than necessary can be prevented fromoccurring.

As the output voltage Vout increases, the timing at which the second onsignal Son2 goes to a high level comes earlier. As a result, thetendency that the timing at which the switching transistor Tr1 turns onis determined by the second on signal Son2 becomes more significant inan area where the output voltage Vout is high. Accordingly, there is anadvantageous effect in that a rapid charging can be achieved by properlysetting the timing of the off-time Toff set by the second on-signalgenerator 96, namely, the timing at which the second on signal Son2 goesto a high level.

FIG. 12 is a circuit diagram showing a modification (60 b) of theswitching control unit according to the fourth embodiment shown in FIG.11. In addition to the structure shown in FIG. 11, the switching controlunit 60 includes a function of monitoring a voltage corresponding to theoutput voltage Vout appearing at the output capacitor C1 and a settingfunction of whether to turn on the switching transistor Tr1 in responseto either one of the first on signal Son1 and the second on signal Son2according to the output voltage Vout.

In addition to the switching control unit 60 a shown in FIG. 11, theswitching control unit 60 b shown in FIG. 12 includes a comparator 68and an AND gate 69. The comparator 68 monitors a voltage Vout′corresponding to the output voltage Vout appearing at the outputcapacitor C1 and then compares the voltage Vout′ against a secondpredetermined threshold voltage Vth2. The voltage Vout′ monitored by thecomparator 68 may be the same voltage as the monitoring voltage Vout′monitored by the charging completion detecting circuit 70 or may bedifferent therefrom. An output signal Sen of the comparator 68 goes to ahigh level when Vout′>Vth2, whereas it goes to a low level whenVout′<Vth2.

The AND gate 69 outputs the logical product of the output signal Sen ofthe comparator 68 and the first on signal Son1. When Vout′>Vth2, thelogical value of an output signal Son1′ of the AND gate 69 is equal tothe logical value of the first on signal Son1. When Vout′<Vth2, thelogical value thereof is fixed to the low level. That is, when theoutput voltage Vout is lower than a predetermined fixed value, thecomparator 68 and the AND gate 69 carry out a function of disabling thefirst on signal Son1. This predetermined value is preferably set to avalue of about 20 V to 50 V.

The OR gate 66 outputs the logical sum of the output signal Son1' of theAND gate 69 and the second on signal Son2 to a set terminal of theflip-flop 62. Other structures and operations are the same as those inthe switching control unit 60 a.

In an area where the output voltage Vout is low, the switching controlunit 60 b of FIG. 12 disables the first on signal Son1 by the AND gate69. However, the generation of the first on signal Son1 may be stoppedby stopping the operation of the first on-signal generator 90 itself.

When the output voltage Vout is lower than a predetermined value, theswitching control unit 60 b disables the switching of the switchingtransistor Tr1 according to the first on signal Son1, and performs theswitching of the switching transistor Tr1 according to the off signalSoff and the second on signal Son2. When the output voltage Vout becomeshigher than the predetermined value, the same circuit operation as thatof the switching control unit 60 a shown in FIG. 11 is performed.

When the output voltage Vout is low, a voltage ΔV appearing across theprimary coil 12 in an off state of the switching transistor Tr1 issmall. Thus, because of the spike noise, ringing or the like, it ispossible that the voltage ΔV becomes lower than the first thresholdvoltage Vth1 even before the energy stored in the transformer 10 isexpended. Further, it is possible that the on-timing of the switchingtransistor Tr1 is adversely affected and thereby the normal circuitoperation is interrupted. In contrast thereto, by employing theswitching control unit 60 b of FIG. 12, the control is performed by thesecond on signal Son2 generated by the second on-signal generator 96when the output voltage Vout is low. As a result, the switchingoperation can be stably performed without the effect of noise and thelike.

Fifth Embodiment

In the first and the fourth embodiment, a description was given of acase where the second on-signal generator 96 takes, as the monitoringvoltage Vout′, a voltage obtained when the output voltage Vout appearingat the output capacitor C1 is voltage-divided by the resistor R2 and theresistor R3 and then sets the off-time Toff and detects the chargingcompletion. If the output voltage Vout rises up to several hundreds ofvoltages, high voltage resistant elements will be required as theresistors R2 and R3, thus causing a problem of an increase in the numberof circuit components. A fifth embodiment described hereinbelow providesa technique wherein the output voltage Vout is monitored with a simplercircuit configuration so as to be reflected in the switching operationof the switching transistor Tr1.

FIG. 13 is a circuit diagram showing a partial structure of a capacitorcharging apparatus 210 according to the fifth embodiment. FIG. 13 showsa circuit configuration which differs from that of FIG. 8 but thecomponents in common with those in FIG. 8 are omitted. In the fifthembodiment, a tap 16 (F winding) is provided on a secondary coil 14 of atransformer 10, and a voltage Vout″ appearing at the tap 16 is inputtedto a voltage monitoring terminal 108. A voltage corresponding to anoutput voltage Vout appearing at the output capacitor C1 appears at thetap 16. More specifically, the voltage is approximately proportional tothe output voltage Vout.

The capacitor charging apparatus 210 according to the fifth embodimenttakes, as a monitoring voltage, the voltage Vout″ appearing at the tap16 of the secondary coil 14 and controls the switching operation of aswitching transistor Tr1. For example, a second on-signal generator 96may set an off-time Toff according to the monitoring voltage Vout″inputted to the voltage monitoring terminal 108. A charging completiondetecting circuit 70 may detect the completion of charging according tothe monitoring voltage Vout″. The switching control unit 60 b shown inFIG. 12 may disable a first on signal Son1 according to the monitoringvoltage Vout″.

According to the fifth embodiment, the voltage in response to the outputvoltage Vout can be detected with accuracy and this accurately detectedvoltage can be reflected in the switching operation of the switchingtransistor Tr1. The tap 16 is provided on the ground side of thesecondary coil 14, namely, on the low voltage side. This eliminates theuse of resistors for voltage division. Or, even if the resistors forvoltage division are required, they need not be any high voltageresistant elements, so that the number of circuit elements used can bereduced.

Sixth Embodiment

In a sixth embodiment, a description will be given of a techniqueconcerning the reduction in size of the capacitor charging apparatus.The transformer 10 described with reference to FIG. 8 or FIG. 13 iscomprised of four terminals or five terminals, respectively. Theincreased number of terminals poses a hindrance to reducing the size incomponents of the transformer 10. In particular, when it is installed ina digital camera or mobile phone terminal, demand for small size is veryhigh among users. A capacitor charging apparatus, described hereinbelow,according to the sixth embodiment reduces the number of terminals of thetransformer 10 and thereby achieves a compact size of the apparatus.

FIG. 14 is a block diagram showing a structure of a capacitor chargingapparatus 210 a according to a sixth embodiment. A structure of atransformer 10 a which is the main feature of this sixth embodiment willfirst be described. The transformer 10 a of FIG. 14 includes a primarycoil 12 a and a secondary coil 14 a. One end of the primary coil 12 a isconnected in common with one end of the secondary coil 14 a, and aterminal P1 is so provided as to form a common connection point thereof.A terminal P2 and a terminal P3 are provided for the other ends of theprimary coil 12 a and the secondary coil 14 a, respectively. Thetransformer 10 shown in FIG. 8 has four terminals, whereas thetransformer 10 a shown in FIG. 14 has three terminals, which is one lessthan those of FIG. 8. As a result, the transformer 10 a of FIG. 14 canachieve a smaller size than the transformer 10 of FIG. 8, so that thecapacitor charging apparatus 210 as a whole can achieve a smaller size.

Next, a general structure of the capacitor charging apparatus 210 ashown in FIG. 14 will be described.

A first terminal P1, where the one end of the primary coil 12 a isconnected in common with the one end of the secondary coil 14 a, isconnected to an input terminal 202, where a battery voltage Vbat isapplied as an input voltage.

A switching transistor Tr1 is connected to a second terminal P2 which isprovided on the other end of the primary coil 12 a of the transformer 10a. An anode of a rectifier diode D1 is connected to a third terminal P3which is provided on the other end of the secondary coil 14 a. An outputcapacitor C1 is provided between a cathode of the rectifier diode D1 andground. A voltage appearing at the output capacitor C1 is outputted froman output terminal 204 as an output voltage Vout. A control circuit 100c generates a switching signal Vsw and controls the on and off of theswitching transistor Tr1 by supplying the switching signal Vsw thereto.

The control circuit 100 c may be configured by using the control circuitshown in FIG. 8 or FIG. 13, a control circuit of other types or aself-excited control circuit incorporating an oscillator therein. In thesixth embodiment, means for turning on and off the switching transistorTr1 is not limited to any particular one.

Now a description will be given of an operation of a capacitor chargingapparatus 210 a configured as described above. As the switchingtransistor Tr1 turns on and off, a primary current Ic1 flows through theprimary coil 12 a of the transformer 10 a from the first terminal P1toward the second terminal P2. At the same time, a secondary current Ic2flows through the secondary coil 14 a from the first terminal P1 towardthe third terminal P3. The output capacitor C1 is charged with thissecondary current Ic2 so as to generate a high-voltage output voltageVout. In the capacitor charging apparatus 210 a according to the sixthembodiment, the transformer 10 a thereof can be made smaller, so thatthe capacitor charging apparatus 210 a can be made smaller than thatshown in FIG. 8.

FIG. 15 is a circuit diagram showing a modification of the capacitorcharging apparatus 210 shown in FIG. 14. In a transformer 10 b shown inFIG. 15, a tap is provided on the secondary coil 14 b and a fourthterminal P4 is provided for this tap. A control circuit 100 d performspart of an on-off control of at least the switching transistor Tr1.

Let the turns ratio of the number of turns in the secondary coil 14 b(from the first terminal P1 through the fourth terminal P4) to thenumber of turns in the primary coil 12 b (from the first terminal P1through the third terminal P3) be expressed as 1:NF. Then, the followingrelations expressed by Equation (1) and Equation (2) hold among a tapvoltage Vtap, a battery voltage Vbat and an output voltage Vout.Vout=Vbat+Vf×NF  (1)Vf=Vtap−Vbat  (2)If Equations (1) and (2) are rearranged and solved for Vtap, Equation(3) will be derived.Vtap=Vout/NF+Vbat×α  (3)The tap voltage Vtap is a voltage directly related to the output Vout.Here, α=(NF−1)/NF. If Vbat is sufficiently small relative to Vout, Vtapcan be approximated as follows.Vtap≈Vout/NF  (4)That is, the control circuit 100 d of FIG. 15 can control the switchingtransistor Tr1 in a manner that the tap voltage Vtap is brought tocorrespondence with the monitoring voltage Vout″ indicated in thecapacitor charging apparatus 210. For instance, the control circuit 100d shown in FIG. 15 may be configured in the same manner as the controlcircuit 100 c shown FIG. 13. That is, the off-time Toff may be setaccording to the tap voltage Vtap. Also, the completion of charging maybe detected according to the tap voltage Vtap. Also, the first on signalSon1 may be disabled according to the tap voltage Vtap.

If the tap voltage Vtap of FIG. 15 is to be monitored, the followingadvantage will be attained compared with the case when the monitoringvoltage Vout″ is monitored. In FIG. 13, one end of the secondary coil 14of the transformer 10 is grounded and set to 0 V. As a result, there arecases where the monitoring voltage Vout″ becomes negative according tothe switching of the switching transistor Tr1. In contrast thereto,since the terminal fixed to the ground voltage (0 V) is fixed to thebattery voltage Vbat, it is advantageous that the tap voltage Vtap ofFIG. 15 will not become negative.

The provision of five terminals is needed for the transformer of FIG.13, whereas four terminals suffice for the transformer 10 b of FIG. 15.Thus, the transformer 10 b can be made smaller in size and at the sametime the size of the capacitor charging apparatus 210 b as a whole canbe made smaller.

Now, a description will be given of a correction of variation in thebattery voltage Vbat. If the term Vbat is not negligible in Equation(3), the tap voltage Vtap will depend on the battery voltage Vbat. Thebattery voltage Vbat varies according to the charging state of thebattery and a consumption degree thereof. In the light of this, thefollowing processing may be performed.

According to an embodiment, the completion of charging is detected bymonitoring the tap voltage Vtap. FIG. 16 is a circuit diagram showing astructure of a charging completion detecting circuit 70 a that detectsthe completion of charging. An input terminal 72 a is connected to thefourth terminal P4 provided in the transformer 10 b, where the tapvoltage Vtap is applied. The charging completion detecting circuit 70 aincludes a difference voltage generation circuit 78, a comparator 74,and resistors R13 and R14.

The difference voltage generation circuit 78 generates a voltage Vxcorresponding to a difference voltage between a terminal 72 a and aterminal 72 b, and then outputs the voltage Vx. The difference voltagegeneration circuit 78 includes resistors R11 and R12, transistors Q1 andQ2, and a constant current source 76. The transistors Q1 and Q2 are eacha PNP bipolar transistor and are connected to each other, in acurrent-mirror configuration, with the bases thereof connected incommon. The resistor R12 is provided between a collector of thetransistor Q1 and ground, whereas the resistor R11 is provided betweenan emitter of the transistor Q1 and the terminal 72 a. The constantcurrent source 76 is provided between a collector of the transistor Q2and ground, whereas an emitter of the transistor Q2 is connected to theterminal 72 b. A voltage Vbat' where a battery voltage Vbat has beensubjected to a voltage division is applied to the terminal 72 b. Thenthe voltage Vx of the collector of the transistor Q1 is given by thefollowing Equation (5).Vx=(Vbat′−Vtap)×R12/R11  (5)Note that the voltage Vx is proportional to the potential differencebetween the terminal 72 a and the terminal 72 b. Note also that asubtractor, configured by the use of an operational amplifier, or othercircuits may be utilized as the difference voltage generation circuit.

The comparator 74 compares the voltage Vx against the threshold voltageVth4. When Vx≧Vth4, the comparator 74 sets a flag FULL indicating thecompletion of charging.

The division ratio determined by the resistor R13 and the resistor R14is so set as to be α=(NF−1)/NF. As a result, the following Equation (6)holds.Vx=(α·Vbat−Vtap)×R12/R11  (6)Put Vx=Vth4 in Equation (6) and Equation (3) and proceed to rearrangethem. Then the following Equation (7) will hold at the time of thecompletion of charging.Vth4=Vout/NF×R12/R11  (7)That is, the output voltage at the time of the completion of charging(hereinafter denoted by VFULL) will be as follows.VFULL=NF×R11/R12×Vth4  (8)In this manner, by employing the charging completion detecting circuit70 a of FIG. 16, the output voltage Vout at the time of the completionof charging can be stabilized to a steady constant value VFULLregardless of the consumption degree of a battery.

The charging completion detecting circuit 70 a generates a voltagecorresponding to the tap voltage Vtap in a manner that a voltage,obtained after the battery voltage Vbat (i.e., input voltage) ismultiplied by a coefficient α, is subtracted from the tap voltage Vtap.Then the charging completion detecting circuit 70 compares this voltageagainst the threshold voltage Vth4 to detect the completion of charging.This is practically equivalent to correcting a threshold value for thecharging completion by the use of the battery voltage Vbat and theconstant α.

The above-described embodiments are merely exemplary, and it isunderstood by those skilled in the art that various furthermodifications to the combination of each component and process thereofare possible and such modifications are also within the scope of thepresent invention.

In the present embodiments, a description has been given of a case wherea bipolar transistor is used as the switching transistor Tr1. However, aMOSFET may be used instead. Also, in the present embodiments, adescription has been given of a case where the capacitor chargingapparatus 210 drives the light-emitting element 212 but this should notbe considered as limiting and it may also be used to drive a variety ofother load circuits requiring high voltage.

Also, the setting of logical values of high level and low leveldescribed in the present embodiments is only one example. The settingcan be changed freely by inverting them as appropriate by an inverter orthe like.

While the preferred embodiments of the present invention have beendescribed using specific terms, such description is for illustrativepurposes only, and it is to be understood that changes and variationsmay be further made without departing from the spirit or scope of theappended claims.

1. A control circuit of a capacitor charging apparatus, the capacitorcharging apparatus including a transformer and an output capacitorcharged with current flowing through a secondary coil of thetransformer, which charges the output capacitor by performing aswitching control of a switching transistor provided on a path leadingto a primary coil of the transformer, the control circuit comprising: aprimary-current detection circuit which detects a primary currentflowing through the primary coil of the transformer; and a switchingcontrol unit which monitors at least the primary current detected bysaid primary-current detection circuit and outputs to a control terminalof the switching transistor a switching signal for specifying ON to theswitching transistor until the primary current reaches a predeterminedpeak current value and then specifying OFF to the switching transistorduring an off-time, wherein said switching control unit adjusts theswitching signal outputted to the control terminal of the switchingtransistor, according to the predetermined peak current value, whereinthe switching transistor is a bipolar transistor, and wherein saidswitching control unit adjusts a current value of a base current, whichis supplied to a base of the bipolar transistor as the switching signal,in accordance with the predetermined peak current value.
 2. A controlcircuit according to claim 1, wherein said switching control unit setsthe current value of a base current in such a manner that the larger thepredetermined peak current value is, the larger the current value of abase current becomes.
 3. A control circuit according to claim 2, whereinsaid switching control unit sets the current value of a base current insuch a manner the current value of a base current is proportional to thepredetermined peak current value.
 4. A control circuit according toclaim 1, wherein said switching control unit includes a currentgenerator which receives a current adjustment signal specifying thepredetermined peak current value and generates a current in accordancewith the current adjustment signal, and wherein the current generated bythe current generator is supplied to a base of the switching transistoras a switching signal in a period during which the switching transistoris to turn on.
 5. A control circuit according to claim 4, wherein thecurrent adjustment signal is inputted to a charge current controlterminal provided in said control circuit, from outside said controlcircuit.
 6. A control circuit according to claim 1, wherein saidswitching control unit includes: a current generator which generates acurrent according to the predetermined peak current value; and a drivercircuit which supplies the current generated by said current generatoras the base current of the bipolar transistor serving as the switchingtransistor, in a period during which the switching transistor is to turnon.
 7. A control circuit according to claim 6, wherein said currentgenerator includes: a voltage to current conversion circuit configuredto receive a current adjustment signal specifying the predetermined peakcurrent value and to generate a current in accordance with the currentadjustment signal; and a current amplifier circuit configured to amplifythe current generated by the voltage to current conversion circuit.
 8. Acontrol circuit according to claim 1, wherein said control circuit isintegrated onto a single semiconductor substrate.
 9. A capacitorcharging apparatus, comprising: a transformer, including a primary coiland a secondary coil, wherein an input voltage is applied to one end ofthe primary coil and a switching transistor is connected to other endthereof; an output capacitor one end of which is grounded; a diodehaving an anode thereof connected to a secondary coil side of saidtransformer and a cathode thereof connected to the other-end side ofsaid output capacitor; and a control circuit, which controls on and offof the switching transistor, wherein the control circuit comprises: aprimary-current detection circuit which detects a primary currentflowing through the primary coil of the transformer; and a switchingcontrol unit which monitors at least the primary current detected bysaid primary-current detection circuit and outputs to a control terminalof the switching transistor a switching signal for specifying ON to theswitching transistor until the primary current reaches a predeterminedpeak current value and then specifying OFF to the switching transistorduring an off-time, wherein said switching control unit adjusts theswitching signal outputted to the control terminal of the switchingtransistor, according to the predetermined peak current value, whereinthe switching transistor is a bipolar transistor, and wherein saidswitching control unit adjusts a current value of a base current, whichis supplied to a base of the bipolar transistor as the switching signal,in accordance with the predetermined peak current value.
 10. A lightemitting apparatus, comprising: a capacitor charging apparatus accordingto claim 9; and a light-emitting element driven by an output voltageappearing at the output capacitor in said capacitor charging apparatus.11. An electronic apparatus, comprising: a light emitting apparatusaccording to claim 10; and a control unit which controls an emittingstate of said light emitting apparatus.
 12. A method for controlling acapacitor charging apparatus, including a transformer and an outputcapacitor charged with current flowing through a secondary coil of thetransformer, which charges the output capacitor by performing aswitching control of a switching transistor provided on a path leadingto a primary coil of the transformer, the method comprising: detecting aprimary current flowing through the primary coil of the transformer; andoutputting to a control terminal of the switching transistor a switchingsignal for specifying ON to the switching transistor until the primarycurrent reaches a predetermined peak current value and then specifyingOFF to the switching transistor during an off-time; and adjusting theswitching signal outputted to the control terminal of the switchingtransistor, according to the predetermined peak current value, whereinthe switching transistor is a bipolar transistor, and a current value ofa base current, which is supplied to a base of the bipolar transistor asthe switching signal, is adjusted in accordance with the predeterminedpeak current value.